Inherited mutations in the Breast Cancer 1 and 2 genes (BRCA1 and BRCA2) confer high penetrance breast and ovarian cancer phenotypes. BRCA1 and BRCA2 maintain genomic integrity through error-free mechanisms of DNA double strand break repair known as homologous recombination. While deficiency in homologous recombination predisposes individuals to malignancy, it also sensitizes cells to commonly used anti-cancer agents. The molecular determinants that influence homologous recombination efficiency at a double strand break however, are largely unknown. We have recently published that BRCA1 is targeted to lysine63-linked ubiquitin (K63-Ub) structures at DNA damage sites along with a K63-Ub-specific deubiquitinating enzyme (DUB), BRCC36, by the ubiquitin-binding protein Rap80. Emerging evidence has revealed that double strand break associated K63-Ub is synthesized by the Ubc13 E2 enzyme in conjunction with the RNF8 and RNF168 E3 ubiquitin ligases. Our preliminary findings demonstrate that opposing activities of ubiquitin synthesis and breakdown potently influence DNA repair mechanisms. This regulatory loop consisting of Ubc13 directed ubiquitin synthesis and BRCC36 directed DUB activities thus provides an attractive target to control DNA repair mechanism. The major goals of this proposal are (1) to test the hypothesis that double strand break associated ubiquitin levels influence DNA end resection (2) to determine how BRCA1 interaction with the core Rap80 complex influences DNA repair, and (3) to determine if BRCA1 and BRCA2 mutant cells are particularly sensitive to deficiency of Rap80-BRCC36 directed DUB activity. We seek to reveal fundamental biological insights into how ubiquitin modifications at DNA double strand breaks influence DNA repair. These studies have considerable relevance to understanding both tumor suppression and responses to anti-cancer chemotherapy.